The changes in isozyme patterns during early ontogeny （from unfertilized eggs to the 1st stage of young crab） of Ponunus trituberculatus were investigated. Eleven isozymes （ LDH, GDH, ADH, SOD, EST, ME, MDH, SDH, GOT, ACP and α-AMY ） were analyzed by vertical slab polyacrylamide gel electrophoresis. The results showed that the electrophoretograms of seven isozymes （ EST,ME,MDH,SDH, GOT, ACP and α-AMY ） exhibited obvious differences among early ontogenic stages of P. trituberculatus, and the complexity of electrophoretogram pattern tended to increase with development in most of the seven isozymic systems. The stage-specific isozymes can be used as the biochemical indices to identify the specificity of a certain developmental stage. No activity of LDH and GDH was detected during early ontogeny of P. trituberculatths （in the present paper）, Both ADH and SOD isozymes had stable expression at every developmental stage of P. trituberculatus, ADH isozyme had one band, and SOD isozyme had two bands. According to the ontogenetic patterns of eleven isozymes, the expression of isozymes during ontogeny of P. trituberculatus was divided into four types. The ontogenetic polymorphism of isozyme and the isozyme activity at the 1st zoeal stage newly after hatching were also discussed in the present paper.